FIG. 1 is a block diagram of a system for Random Access (RA) in LTE for performing the RA signaling described in FIG. 2. System 10 includes one or more wireless devices 11a-n (referred to collectively as wireless devices 11) and one or more network nodes 12. In modern cellular radio systems such as system 10, the radio network has a strict control on the behavior of wireless device 11. Uplink transmission parameters like frequency, timing, and power are regulated via downlink control signaling from the base station to the terminal. For instance, by time-aligning the uplink (UL) transmissions, orthogonality between wireless devices can be achieved in the time domain, and this is necessary since the radio resources are scarce. UL as used herein refers to transmission from the wireless device to the network node.
At power-on or after a long standby time, the wireless device 11, e.g., User Equipment (UE), terminal, etc., is not synchronized in the uplink. The wireless device can derive an uplink frequency and power estimate from the downlink (control) signals. However, a timing estimate is difficult to make since the round-trip propagation delay between the network node 12, e.g., base station, eNodeB, etc., and the wireless device 11 is unknown. So even if the wireless device uplink timing is synchronized to the downlink, it may arrive too late at the receiver of network node 12 because of the propagation delays. Therefore, before commencing traffic, the wireless device has to carry out a Random Access (RA) procedure to the network. After the RA, the network node can estimate the timing misalignment of wireless device 11 uplink and send a correction message.
Usually, a Physical Random Access Channel (PRACH) is provided for wireless device 11 to request access to the network. A RA preamble is used which is based on a specific sequence with good auto-correlation. Because multiple wireless devices 11 can request access at the same time, collisions may occur between requesting wireless devices. A contention resolution scheme has to be implemented to separate wireless device 11 transmissions. To distinguish between different wireless devices 11 performing RA, typically many different preambles exist. Wireless device 11 performing RA randomly picks a preamble out of a pool and transmits it. The preamble represents a random wireless device ID which can be used by network node 12 when granting the wireless device access to the network. Network node 12 receiver can resolve RA attempts performed with different preambles and send a response message to each wireless device 11 using the corresponding random wireless device IDs. In case where multiple wireless devices 11 simultaneously use the same preamble, a collision occurs, and the RA attempts will most likely not be not successful since network node 12 cannot distinguish between the two wireless devices with the same random wireless device ID. To minimize the probability of collision, the set of available sequences should be large.
FIG. 2 is a block diagram of random access signaling in LTE as defined in 3GPP TS 36.321 and 36.213. Wireless device 11 transmits an RA preamble message, i.e., message 1 or Msg1, to network node 12 (Block S100). Network node 12, transmits an RA response message (Block S102). The RA response message includes a timing advance, uplink grant and other information known in the art as defined in 3GPP TS 36.321 and 36.213. Wireless device 11 transmits an RA message 3, i.e., msg3, to network node 12 (Block S104). The RA message 3 may include a wireless device identity, buffer status report (BSR), and other information known in the art as defined in 3GPP TS 36.321 and 36.213. Network node 12 transmits an RA contention resolution message to wireless device 11 (Block S106). The RA contention resolution message may include an UL grant, DL assignment and other information defined in 3GPP TS 36.321 and 36.213. Wireless device 11 and network node 12 participate in further uplink and/or downlink transmission (Block S108).
Network node 12 receiver listens at all RA opportunities to detect preambles. In case a preamble is successfully detected, a RA response that includes, e.g., the number of the detected preamble, timing advance information and UL grant for an UL transmission (e.g., Msg3 in step 3 of the RA procedure), is sent in a special message on the downlink (DL). The UL grant included in a RA response is henceforth referred to as an RA response grant.
A wireless device 11 that has recently performed a RA preamble transmission is listening within a certain time window after the preamble has been sent to receive a RA response. In case of a successful reception of the RA response, wireless device 11 continues with Blocks S104 and S106 of the RA procedure. In case no RA response is received within the specified window, a new attempt is made.
After receiving the RA Response, wireless device 11 decodes the message and reads the enclosed RA Response grant. Wireless device 11 then sends the RA msg3 using this grant. In LTE, the timing of the grant is given by the standard and a flag inside the grant.
In LTE, wireless device 11 shall, according to the UL grant information in the RA response, transmit an UL-SCH transport block in the first subframe n+k1, k1≥6, if the UL delay field is set to zero where n+k1 is the first available UL subframe for PUSCH transmission. Wireless device 10 shall postpone the PUSCH transmission to the next available UL subframe after n+k1 if the field is set to 1.